Inorganic nanocrystals exhibit size and shape-dependant physicochemical properties that can be broadly exploited as bioactive agents (Pernodet, N. et al., “Adverse Effects of Citrate/Gold Nanoparticles on Human Dermal Fibroblasts”, Small, Vol. 2, No. 6, Pages 766-773, 2006), catalysts (Hughes, M. D. et al., “Tunable Gold Catalysts for Selective Hydrocarbon Oxidation Under Mild Conditions”, Nature, Vol. 437, No. 7062, Pages 1132-1135, 2005), biosensors (Siwy, Z. et al., “Protein Biosensors Based on Biofunctionalized Conical Gold Nanotubes”, J. Am. Chem. Soc., Vol. 127, No. 14, Pages 5000-5001, 2005), data storage (Sun, S. et al., Science, Vol. 287, 1989-1992, 2000) and optics (Wang, J. F. et al, Science, Vol. 293, 1455-1457, 2001). There is a strong interest in controlling the synthesis and morphology of metal structures on the nanoscale. Several methods for nanostructure synthesis are now available with, in some cases, control over their composition and architecture. Faraday described the formation of gold colloids nearly 150 years ago (Faraday, M., Phil. Trans. Roy. Soc., Vol. 147, Page 145, 1857). Boiling gold salts with citric acid (Turkevich, J., et al., Disc. Faraday Soc. Vol. 11, Page 55, 1951) gives primarily pyramidal particles (Thomas, J. M., “Colloidal Metals: Past, Present and Future”, Vol. 60, No. 10, Pages 1517-1528, 1988). A series of different strategies including the use of surfactants, high UV and chemical vapor deposition (CVD) are being exploited in an attempt to gain more control over the preparation of metallic structures. Generic routes to a broad range of nanostructures for specific applications, using the same set of synthetic tools, have proven elusive.
There are a wide variety of applications that require control of the interfacial properties between immiscible components, such as water-in-oil emulsions or oil-in-water emulsions. Generally, to obtain good performance, it is necessary to stabilize the interface between the two immiscible components. One simple example is the use of coupling agents to modify silica surfaces so that silica may be used to reinforce organic polymers, with which it is otherwise incompatible (Plueddeman, E. P., Silane Coupling Agents, Plenum Press: New York, 2nd Ed. 1991). Another example is the use of surfactants to stabilize oils in water, such as in cleaning and conditioning applications.
Silicones are among the most surface-active materials (“surfactants”) known. They diffuse rapidly to interfaces and readily spread. Spreading of the silicone may be facilitated by the incorporation of polar groups on the silicone backbone. Some of the most effective spreading compounds, particularly at solid/liquid/air surfaces, are the so-called “superwetters” made by manufacturers including Crompton Corp. and Dow Corning. The general structure of these superwetters is ((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)nOZ, where Z may be H, CH3, CH3COO, etc. (Hill, R. M., Silicone Surfactants, Dekker, 1999).
Liquid-liquid interfaces are generally stabilized with silicones bearing non-ionic hydrophilic groups. Common examples include derivatives of so-called silicone polyols; that is silicones containing polyether sidechains. U.S. Pat. No. 5,707,613 issued to Hill teaches that these compounds are particularly useful at stabilizing water/silicone interfaces. Ionic silicone copolymers can also be used to stabilize such interfaces. U.S. Pat. No. 5,124,466 issued to Azechi et al. teaches that ammonium-modified silicone surfactants are useful in the stabilization of silicone emulsions in water. Anionic silicone surfactants are also known. U.S. Pat. No. 5,447,997 issued to Releigh et al. teaches silicones containing carboxylic acids whose surface properties change as a function of pH.
The surface activity of silicones, whether cationic, anionic, zwitterionic or non-ionic, cannot be readily changed, although pH modifications may affect the behavior of some types of ammonium compounds at high pH or carboxylic acids and other acids at low pH. There are advantages in being able to change the surface activity of a surface active material so as to change the properties of the system in accordance with its particular use, for example, to flocculate emulsions on demand. For example, carboxylic acids and polymers derived therefrom (e.g., CARBOPOL™ (available from BF Goodrich)) can swell in water and stabilize interfaces upon pH changes in such instances as when bases convert neutral carboxylic acids to carboxylates. In this respect, silicones having a pH sensitivity, by virtue of amine or carboxylic acid groups, are known.
The properties of ionic surfactants may not only be changed by pH, but by the nature of the counterions as well. For example, carboxylates with monovalent counterions such as sodium swell well with water. In contrast, the presence of multivalent counterions in the same system lead to ionic crosslinking and a reduction of swelling. At an interface, the surface activity of such materials is similarly affected by the nature of the counterion.
Multidentate ligands (or “chelating agents”) bind metals very tightly. The classic example is EDTA (ethylenediaminetetraacetic acid). EDTA, normally in its calcium, disodium salt form, is frequently found in food products. Heavy metal ions coming into contact with the EDTA will complex with the amine and carboxylic acid groups, displacing the sodium/calcium ions. The binding efficiency of EDTA and its derivatives is known for many metals and their different oxidation states. Chelating agents are added to many different formulations for different purposes. They have also been bound to polymers. For example, chelating groups similar to those mentioned above are used as supports in affinity chromatography.
U.S. Pat. No. 6,566,322 issued to R. S. Stan and M. A. Brook describes the use of silicones capable of chelating metals. In this case, nitrilotriacetic acid derivatives were added to linear silicones either at the termini, or pendant along the chain. While these materials offer advantageous properties, there remains a need to develop materials with such properties and the additional ability to undergo redox chemistry.